A number of other investigators have suggested that measles virus may play a pathogenetic role in multiple sclerosis (MS). In this study, measles-specific human T cell clones will be derived from lymphocytes obtained from patients with MS and from controls. Peripheral blood lymphocytes will be cultured with antigen, cloned by limiting dilution, and expanded in the presence of antigen, autologous irradiated feeder cells, and interleukin-2. The clones will be characterized in terms of proliferation, cytolytic activity, helper and suppressor activity, antigen-specificity, surface phenotype, lymphokine production, antigen-binding, and HLA-restrictions. The clones will be used to study the regulation of the immune response to measles virus in MS and control populations. This will be done primarily with cloned and uncloned cell populations derived from three sets of twins who are HLA-identical but discordant for both MS and for measles reactivity. Cell mixing experiments will be performed to examine clone-clone interactions as well as interactions between clones from one subject and uncloned T cell subsets or phagocytic cells from the other subject. The HLA-identity of the subjects will allow optimal cell-cell interactions in this antigen-specific system. Reactivity against purified measles antigens (e.g. hemagglutinin, fusion protein, etc.) will be examined and compared between the two populations. These studies will allow us to determine the cellular level at which measles-responsiveness or unresponsiveness is established in these subjects. Anti-idiotypic antibodies will be produced against the clones. These wil eventually be used in the detection, isolation, and study of idiotype-bearing molecules on the cell surface and in secreted products.